Coating aluminum surfaces



United States Patent 3,039,898 COATING ALUMINUM SURFACES Fred Keller, New Kensington, and Walter G. Zelley, Lower Burrell, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsyl- Vania No Drawing. Filed Feb. 3, 1961, Ser. No. 86,866 4 Claims. (Cl. 1486.14)

This invention relates to the treatment of aluminum surfaces, and more particularly to an improved method for imparting a protective coating or film to such surfaces. This application is a continuation-in-part of our co-pending applications Serial Nos. 852,932 and 30,430, filed November 16, 1959, and May 20, 1960, respectively, both now abandoned. As used herein, the word aluminum includes pure aluminum, commercial aluminum containing the usual impurities, and aluminum base alloys.

Aluminum surfaces provided with artificial oxide coatings or films may afiord high insulating properties, and increased resistance against corrosion, chemical action and the like. Such protective oxide coatings or films may be produced commercially by chemical or electrochemical methods. For example, an oxide coating may be produced by treating the aluminum surface with an alkaline solution of a carbonate, usually a sodium carbonate-chromate solution. In another common method, an oxide coating may be produced by the anodic oxidation of the aluminum surface whereby the aluminum is made the anode is an electrolytic solution such as sulfuric acid solution or chromic acid solution. However, oxide coatings formed by these well-known commercial methods are amorphous, relatively porous and absorbent. The anodic coatings generally are sealed as by immersing the aluminum article in boiling water or a boiling nickel acetate solution. Sealing renders the oxide coating nonabsorbent thereby minimizing corrosion or staining of the coating. In order to aflord sufficient protection with such coatings, it is essential to produce a relatively thick oxide film, and sometimes anodic oxide coatings are made as thick as about one mil or more. However, relatively thick, hard coatings, being somewhat brittle, have a tendency to craze as, for example, when Wound in a small radius or bent at an acute angle, and therefore are somewhat limited in applications.

Another known method for producing an oxide film on an aluminum surface comprises treating the surface with boiling water usually for one or more hours, and often as much as three hours or even longer. Oxide films formed in this manner are hard, crystalline films of tic-alumina monohydrate, represented by the formula Al O -H O. Relatively thin conversion films of this nature exhibit high resistance to corrosion or chemical attack and high electrical insulating properties. For most applications, the thickness of the film should not be less than about 0.01 mil. However, use of a-alumina monohydrate surface films has attained only very limited commercial success because of the prolonged treatment required for forming a film of suflicient thickness.

In our co-pending United States application entitled Treating Aluminum Surfaces, Serial No. 86,867, filed concurrently herewith, we describe producing an a-alumina monohydrate film on an aluminum surface by immersing the same in an aqueous bath of certain slightly soluble alkaline earth compounds. In such process the oxide film may be formed in a relatively short period of time by maintaining the temperature of the bath at 175 F. or above. We have found, quite unexpectedly, that substantially thicker oxide coatings may be produced on an aluminum surface by immersing the same in a solution consisting essentially of water and magnesium oxide. Further, such a solution may be employed over a Wider temperature range than is referred to in our concurrently filed application. The increased thickness of the coating is attributable to the presence of hydrated magnesium oxide, often in association with some hydrated aluminum oxides. Coatings of this nature show high electrical insulating properties. The hydrated magnesium oxide is generally found present in an amount of at least about 20% by Weight of the coating, and may even comprise the major part of the coating. For suitable electrical insulating applications, the total thickness of the coating should not be less than about 0.1 mil and may be higher.

It is, therefore, the primary object of this invention to provide an improved method of imparting a protective coating or film to an aluminum surface in a relatively short period of time by employing a chemical method.

It is another object of the invention to form a coating or film comprising hydrated magnesium oxide on an aluminum surface, which coating or film is characterized by high insulating properties.

In accordance with the present invention, an oxide coating may be produced on an aluminum surface by immersing the same-in a Warm or hot solution consisting essentially of water and magnesium oxide, containing not less than about 0.5 gram per liter of magnesium oxide so that such compound is present in the solution in an amount in excess of its solubility in the solution. Solutions of less than about 0.5 gram per liter of magnesium oxide do not produce a coating of the thickness usually desired. On the other hand, generallyno substantial benefit appears to be gained by employing more than about 10 grams per liter of magnesium oxide. For best results, and for relative ease of operation, the solution preferably contains about 3 to 6 grams per liter of magnesium oxide. A dense, crystalline oxide film comprising hydrated magnesium oxide, of thickness ordinarily desired, may be so imparted to the aluminum surface by a single immersion in the solution for at least about 1 minute.

It has been found that the solution employed in accordance with our procedure for producing the coating has a pH of not less than about 8.5 and may be as high as 11, a pH above 10 being particularly desirable for carrying out the process in a short period of time. Generally, treating aluminum surfaces in a solution having a pH lower than approximately 8.5 proceeds too slowly to be economically feasible, particularly for continuous coating operations. On the other hand, since the alkalinity of the solution normally does not exceed a pH of about 11, any substantial etching action resulting in dissolving away of the aluminum metal and oxide that forms is avoided.

It is be observed that because magnesium oxide is only slightly soluble in water, even at elevated temperatures, the relatively low concentration of the compound in solution renders the aqueous solution alkaline within the pH range found desirable for the practice of our invention, thereby obviating the need for a buffer. Magnesium oxide has a maximum solubility of about 0.09 gram per liter in hot water. A solution of magnesium oxide has a pH of about 9.5 or higher, at 150 R, which may be readily maintained by the desired excess of undissolved magnesium oxide. Thus, employing only a single compound n low concentrations results in substantial economic savings.

According to our invention, the desired oxide coating can but be obtained #by treating the aluminum surface in a bath as herein described, which is maintained at a temperature above about -F., and preferably between about F. and 200 F. Employing bath temperatu-res lower than about 125 F. extends considerably the time required for producing the oxide coating (or producies undesirably thin films) and, therefore, is not practica Immersing an aluminum article in the aqueous solution of magnesium oxide for about 1 to 15 minutes, and preferably 3 to 12 minutes, will produce the desired crystalline oxide film, when operating within the above-described temperature ranges. More prolonged immersions may increaseslightly the thickness of oxide coating produced, but this increase generally is not suificient to warrant the longer irnmersions. On the other hand, immersions for less than about 1 minute may in some cases be useful, but generally are not sufiicient to produce an oxide surface coating of depth usually desired.

Aluminum panels treated in accordance with our invention were tested for the DC. voltage breakdown of the oxide coating to determine the insulating properties of the coating. The voltage breakdown tester was that developed by the National Bureau of Standards and published in The Review of Scientific Instruments, vol. 24, No. 6, p. 458 (June 1953), employing a one ounce copper probe. The coatings of hydrated magnesium oxide formed on the treated panels had thicknesses between about 0.11 to 0.16 mil. The results are set forth in the Aluminum panels having a natural oxide film exhibit an average voltage breakdown of less than 10.

The hydrated magnesium oxide coating formed on an aluminum surface in accordance with our invention is characterized by high insulating properties. Aluminum strip provided with this film is useful, for example, in electrical condensers or foil Wound coils where windings of small radii would not craze the thin oxide film.

Having described our invention, we claim:

1. The method of producing an oxide coating on an aluminum surface, comprising immersing an aluminum surface, for at least about 1 minute, in a solution maintained at a temperature above about 125 and consisting essentially of water and at least about 0.5 gram per liter of magnesium oxide, said magnesium oxide rendering the solution alkaline within a pH range of approximately 8.5 to 11, whereby a crystalline coating comprising hydrated magnesium oxide is formed on the aluminum surface.

2. The method of claim 1 wherein the solution contains about 3 to 10 grams per liter of magnesium oxide.

3. The method of claim 1 wherein the solution is maintained at a temperature between about 140 F. and 200 F.

4. The method of claim 1 wherein the aluminum surface is immersed for about 1 to 15 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 1,846,844 Clark Feb. 23, 1932 2,129,840 Hessenbruch Sept. 13, 1938 2,250,472 -De Long July 29, 1941 2,859,148 Altenpohl Nov. 4, 1958 

1. THE METHOD OF PRODUCING AN OXIDE COARTING ALUMINUM SURFACE, COMPRISING IMMERSING AN ALUMINUM SURFACE, FOR AT LEAST ABOUT 1 MINUTE, IN A SOLUTION MAINTAINED AT A TEMPERATURE ABOVE ABOUT 152*F. AND CONSISTING ESSENTIALLY OF WATER AND AT LEAST ABOUT 0.5 GRAM PER LITER OF MAGNESIUM OXIDE, SAID MAGNESIUM OXIDE RENDERING THE SOLUTION ALKALINE WITHIN A PH RANGE OF APPROXIMATELY 8.5 TO 11, WHEREBY A CRYSTALLINE COATING COMPRISING HYDRATED MAGNESIUM OXIDE IS FORMED ON THE ALUMINUM SURFACE. 